1. Field of the Invention
The present invention generally relates to improvements in telephone communication systems having a plurality of extension telephones adapted to be interconnected with the same telephone line and, more particularly, to a telephone intruder alarm system for and a method of indicating the subsequent interconnection and disconnection of an extension phone with a telephone line to a current on-line user. Still more particularly, the invention relates to a method of installing such systems on conventional telephone lines.
2. Description of the Prior Art
In conventional telephonic communication systems, extension telephones are adapted to be connected in parallel with each other and with a telephone line through appropriate switches located in the base of the respective telephones. That is, an extension telephone is connected with a telephone line simply by removing the receiver from the cradle so that the particular telephone goes "OFF-hook." The person using the telephone may then place his call and converse with the called party in the conventional manner. Additionally, any one of the other extension telephones may also be connected with the same telephone line by the same procedure (i.e. by removing the receiver of any one of the other extension telephones from its associated cradle). In fact, this type of system has been used extensively for conference calls in which a plurality of people are connected with the called or calling party, as the case may be.
However, this same system also provides a means for allowing unauthorized personnel to overhear private conversations. More specifically, someone desiring to eavesdrop on another party's conversation may do so by removing the receiver of any one of the unused extension telephones. Consequently, parties desiring complete telephonic privacy have been forced to use separate phone lines, rather than risk the chance of being overheard, either accidentally or otherwise, through an unused extension telephone.
In order to eliminate the possibility of such eavesdropping by unauthorized personnel, the prior art has proposed a multi-element telephone exclusion circuit arrangement which is operative to connect a selected one of a plurality of extension telephones to a telephone line and to automatically exclude all other extension telephones therefrom. Although the proposed telephone exclusion circuit arrangement is generally satisfactory for its intended purpose, it is not altogether practical, nor cost effective in practice. The complexity, high cost and maintenance problems associated with a telephone exclusion circuit which is comprised of a large number of parts, including separate wiring, makes it particularly undesirable for use in the home environment.
In U.S. Pat. No. 4,136,266, a novel highly reliable system which overcomes the drawbacks mentioned above and which alerts an on-line user to the presence of an intruder on the telephone line is set forth. Although very satisfactory for its intended purpose, it is now desirable to further refine this type of system.